FIG. 1 shows an exemplary network structure of a Long-Term Evolution (LTE) system as a mobile communication system to which a related art and the present invention are applied. The LTE system is a system that has evolved from the existing UMTS system, and its standardization work is currently being performed by the 3GPP standards organization.
The LTE network can roughly be divided into an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) and a Core Network (CN). The E-UTRAN is generally comprised of a terminal (i.e., User Equipment (UE)), a base station (i.e., Evolved Node B (eNode B)), an access gateway (aGW) that is located at an end of the network and connects with one or more external networks. The access gateway may be divided into a part that handles processing of user traffic and a part that handles control traffic. In this case, the access gateway part that processes the user traffic and the access gateway part that processes the control traffic may communicate with a new interface. One or more cells may exist in a single eNB. An interface may be used for transmitting user traffic or control traffic between eNBs. The CN may include the aGW and a node or the like for user registration of the UE. An interface for discriminating the E-UTRAN and the CN may be used.
FIGS. 2 and 3 show respective exemplary structures of a radio interface protocol between the terminal and the E-UTRAN based on the 3GPP radio access network standards. The radio interface protocol has horizontal layers comprising a physical layer, a data link layer, and a network layer, and has vertical planes comprising a user plane (U-plane) for transmitting user data information and a control plane (C-plane) for transmitting control signaling. The protocol layers in FIGS. 2 and 3 can be classified into a first layer (L1), a second layer (L2), and a third layer (L3) based on three lower layers of an open system interconnection (OSI) standard model widely known in the communication system. The radio protocol layers exist as pairs between the UE and the E-UTRAN and handle a data transmission in a radio interface.
The layers of the radio protocol control plane in FIG. 2 and those of the radio protocol user plane in FIG. 3 will be described as follows.
The physical layer, the first layer, provides an information transfer service to an upper layer by using a physical channel. The physical layer is connected to an upper layer called a medium access control (MAC) layer via a transport channel. Data is transferred between the MAC layer and the physical layer via the transport channel. The transport channel is divided into a dedicated transport channel and a common transport channel according to whether or not a channel is shared. Between different physical layers, namely, between a physical layer of a transmitting side and that of a receiving side, data is transmitted via the physical channel using radio resources.
The second layer includes various layers. First, a medium access control (MAC) layer performs mapping various logical channels to various transport channels and performs logical channel multiplexing by mapping several logical channels to a single transport channel. The MAC layer is connected to an upper layer called a radio link control (RLC) layer by a logical channel. The logical channel is roughly divided into a control channel that transmits information of the control plane and a traffic channel that transmits information of the user plane according to a type of transmitted information.
A Radio Link Control (RLC) layer of the second layer segments and/or concatenates data received from an upper layer to adjust the data size so as for a lower layer to suitably transmit the data to a radio interface. In addition, in order to guarantee various Quality of Services (QoSs) required by each radio bearer (RB), the RLC layer provides three operational modes: a Transparent Mode (TM); an Unacknowledged Mode (UM); and an Acknowledged Mode (AM). In particular, the AM RLC performs a retransmission function through an Automatic Repeat and Request (ARQ) for a reliable data transmission.
A Packet Data Convergence Protocol (PDCP) layer of the second layer performs a function called header compression that reduces the size of a header of an IP packet, which is relatively large and includes unnecessary control information, in order to effectively transmit the IP packet such as an IPv4 or IPv6 in a radio interface having a narrow bandwidth. The header compression increases transmission efficiency between radio interfaces by allowing the header part of the data to transmit only the essential information. In addition, the PDCP layer performs a security function in the LTE system. The security function includes ciphering for preventing data wiretapping by a third party, and integrity protection for preventing data manipulation by a third party.
The Radio Resource Control (RRC) layer located at the lowermost portion of the third layer is defined only in the control plane, and controls a logical channel, a transport channel and a physical channel in relation to the configuration, reconfiguration, and release of radio bearers (RBs). In this case, the RBs refer to a logical path provided by the first and second layers of the radio protocol for data transmission between the UE and the UTRAN. In general, configuration (establishment, setup) of the RB refers to the process of stipulating the characteristics of a radio protocol layer and a channel required for providing a particular data service, and setting the respective detailed parameters and operational methods. The RBs include two types: a Signaling RB (SRB) and a Data RB (DRB). The SRB is used as a path for transmitting an RRC message on a C-plane, and the DRB is used as a path for transmitting user data on a U-plane.
In the related art, if a RLC (Radio Link Control) layer receives a negative acknowledgement (NACK) (i.e., delivery failure notification) with respect to a transmission of a particular RLC data PDU, the RLC layer retransmits a corresponding RLC data PDU after checking sequence number(s) of the corresponding RLC data PDU.
However, in a certain situation, there is case that the RLC layer receives a HARQ delivery failure notification with respect to a RLC data PDU that was received positively (i.e., ACK) based on a previous RLC status PDU. This case happens when there is an ACK-to-NACK error during a HARQ operation. In this case, the RLC data PDU must be retransmitted according to the related art because the retransmission of the RLC data PDU is basically determined by a report of the RLC status PDU. However, such retransmission of the RLC data PDU is unnecessary in case that there is an ACK-to-NACK error during the HARQ operation. As such, radio resources can be wasted because of such unnecessary retransmission of the RLC data PDU. Further, in the related art, if the acknowledged RLC data PDU by a RLC status PDU is erased from a transmitting side buffer, a retransmission of the RLC data PDU can not possibly performed even if the RLC receives the HARQ delivery failure notification. From this, the RLC protocol can be in a deadlock situation, which causes a great drawback in the related art.